Bulimia nervosa is one of the most common psychiatric problems faced by female adolescents. Its persistent course can result in serious medical complications, and is associated with co-morbid psychiatric disorders. Although several psycho-educational eating disorder prevention programs have been evaluated in controlled trials, none have reduced bulimic symptoms. The proposed project represents a large-scale randomized trial of a new type of eating disorder prevention program that, in a controlled pilot study (N= 117), resulted in significant reductions in endorsement of the current thin-ideal for women (thin-ideal internalization), body dissatisfaction, dieting, negative affect, and bulimic symptoms). This new program is a dissonance-based intervention, wherein high-risk females who show elevations in thin-ideal internalization voluntarily engage in a series of verbal, written, and behavioral exercises requiring them to critique the thin-ideal during 3 weekly sessions. This intervention is the outgrowth of an 8-year program of generative research on the risk factors for bulimic pathology by this research team. The proposed study will randomly assign 450 high-risk females to the dissonance condition, a psycho-educational placebo control condition, or a measurement-only condition and will follow them for 2 years after program termination. The aims of this study are to test whether (1) the dissonance intervention results in significantly greater reductions in bulimic symptoms than the placebo and measurement-only control interventions, (2) the program effects on bulimic symptoms are mediated by change in thin-ideal internalization, body dissatisfaction, dieting, and negative affect, and (3) certain factors potentiate (e.g., elevated distress regarding body image and eating disturbances) or mitigate (e.g., poor insight regarding the adverse effects of pursuit of the thin-ideal) program effects. Not only does this intervention appear to be the first to effectively reduce bulimic symptoms, but the proposed study is more methodologically rigorous than past trials because of the use of random assignment, an active placebo control condition, structured psychiatric interviews for assessment purposes, and a long-term follow-up period. Thus, the proposed study should be the definitive test of this new promising eating disorder prevention program and should significantly advance our knowledge regarding the prevention of this pervasive psychiatric problem.